This invention relates generally to wide-area cold cathode electron guns and x-ray generators and more specifically to electrodes therefor that utilize a corona plasma for liberating electrons. Commercial x-ray generators used in medical applications, materials testing, airport security, etc. are high-vacuum electron tubes with thermionic cathodes, producing electron beam pulses of tens of microseconds duration and having current densities of a few mA/cm.sup.2. The electron beam (e-beam) is focused on a small, high-Z target area on the anode, which emits the x-rays.
New applications for electron beam and x-ray pulses in the field of high energy lasers require space-charge limited e-beam current densities in the range of 1 to 10 A/cm.sup.2, short pulses of less than 1 microsecond duration L and high intensity irradiation over wide areas on the order of 10.sup.2 to 10.sup.4 cm.sup.2. A number of cold cathode guns have been developed to fill this need. However, most of these guns suffer from a short cathode life, typically limited to 10.sup.3 to 10.sup.7 pulses.
Electrically pulsed, self-sustained discharge-excited, high-energy gas lasers, such as XeCl, XeF, KrF, C0.sub.2, and HgBr lasers, require uniform preionization of the high pressure gas mixture for discharge uniformity and stability. Generally, an ionization level of at least 10.sup.6 electrons/cm.sup.3 is necessary during the avalanche discharge formation process to prevent the electron avalanche from developing into a set of narrow streamers or arcs. See A. J. Palmer, Appl. Phys Lett. 25 (1974) 138 and J. I. Levatter and S. C. Lin, "Necessary Conditions for the Homogeneous Formation of Pulsed Avalanche Discharges at High Gas Pressures", J. Appl. Phys. 51 (1980) 210. For optimal laser efficiency and laser beam uniformity, the preionization level should be between 10.sup.8 and 10.sup.9 electrons per cm.sup.3. Pre-ionization uniformity should be within plus or minus twenty percent throughout the discharge volume. Hence, x-ray flux uniformity should also be within plus or minus twenty percent.
Ultraviolet (UV) light from spark arrays, located near the discharge electrodes, is presently utilized in most commercial high-pressure gas lasers to ionize the discharge volume. It is well known, however, that x-ray preionization is superior to the more conventional UV-preionization when the gas laser has a large discharge volume, operates at a high pressure, or is required to have a long gas life and system life of more than 10.sup.8 shots. Under these circumstances, the advantages of x-rays over UV-radiation justify the higher cost of the x-ray generator.
X-ray preionization offers the following advantages: (1) X-rays can preionize large volumes uniformly; (2) The x-ray generator is physically separated from the laser gas, which in many cases is corrosive; (3) The x-ray source does not contaminate the laser as UV sparks do; and (4) The x-ray flux can be collimated or vignetted and passed through one of the discharge electrodes (e.g. a 3 mm thick aluminum window). The x-rays can thus be used to define the discharge dimensions by spatially selective preionization.
Electron guns that are presently available for generating the x-ray pulse differ primarily in the manner in which electrons are liberated in sufficient quantity at the cathode. Hot cathodes, such as thoriated tungsten and dispenser cathodes, require hard vacuum and large heater power. Thermionically limited emitters are limited to a current density of less than 100 mA/cm.sup.2. Cold field-emission cathodes, such as sharp blades, needles, and carbon felt, are simple and rugged, but exhibit very non-uniform emission characteristics after 10.sup.6 pulses, and occasionally after only 10.sup.3 pulses. A carbon felt cathode design with a control grid has recently been patented by Birnbach et al. as U.S. Pat. No. 4,670,894 issued June 2, 1987. Such a cathode, however, is not suitable for pulse repetition rates above a few hertz. See S. J. Scott, "Experimental Investigations on an X-ray Preionizer Test Bed," J. Appl. Phys. 64 (1988) 537.
Only so-called plasma cathode guns have demonstrated lifetimes in excess of 10.sup.8 pulses, when operated in the short pulse (less than 1 microsecond) and high current density (greater than 1 A/cm.sup.2) regime. With proper anode cooling, plasma cathode guns can also be operated at high pulse repetition rates of more than 100 Hz.
A grid controlled plasma gun which liberates electrons from the cathode by secondary electron emission due to positive ion bombardment is known as the "Wire Ion Plasma" or WIP gun. See G. Wakalopulos, "High Peak Power Pulsed WIP Electron Gun," Final Report, Contract No. 78-73-09176/E1377-002 Lawrence Livermore Radiation Lab., Livermore, Calif. (October, 1978). It can be built quite rugged, and can operate at pulse rates above 10 KHz. Its major drawbacks are high complexity, high cost, slow current rise, and the need for accurate control of the background gas pressure.
A surface-spark-cathode gun has recently been developed and marketed by Beta Development Corp., Dublin, Calif. The electron source is a spark plasma created by a series of surface discharges initiated by a 10 kV pulse. The electrons are then accelerated toward an anode maintained at 70 kV to 100 kV DC. Because of the small amount of energy (approximately 1 mJ) switched into the sparks, this cathode has a surprisingly long life of approximately 10.sup.7 pulses. Disadvantages of this gun are a relatively short cathode life of less than 10.sup.8 pulses and a large (30 cm diameter) gun chamber to accommodate the high DC voltage on the anode.
In 1982, a corona plasma cathode was developed by Helionetics, Inc. (HLX Laser), which cathode consisted of a tungsten wire coil wound around a glass tube. European Patent Application No. 0101043 filed Aug. 8, 1983; and J. I. Levatter, R. L. Sandstrom, and J. H. Morris, "The Corona Plasma Cathode: A New Long-Life E-Beam Cathode for X-ray Preionization," Proc. IEEE, 4th Pulsed Power Conf. 1983, ed. T. K. Martin, M. F. Rose (N.Y. 1983). The e-beam anode was a "forward emitting" gold foil target at ground potential. When a high negative voltage pulse (accelerating potential) was applied to the wire coil on the cathode, the electric field between it and a grounded conductor inside the glass tube initiated a surface corona on the outside of the dielectric tube (electron source). Because no high DC voltages have to be sustained by the system, this type of gun can be made quite compact. This corona cathode has a more uniform emission than a carbon felt cathode and has a potential lifetime of more than 10.sup.8 pulses. Pulse rates as high as 200 Hz are possible with this gun, although cooling becomes a problem. The Helionetics gun has several severe drawbacks. One of these drawbacks is that the cathode is subject to catastrophic failure due to cracking or electric break-through of the highly stressed pyrex tube. Another drawback is that non-uniform erosion of the dielectric tube tends to make the electron emission less uniform as the tube ages. A further drawback is that the x-ray generator relies on the less efficient x-ray emission in the forward direction from a high-Z foil or coating on the x-ray window. Anode cooling becomes a problem at high pulse rates.